The present invention relates to a trouble detecting apparatus for the nuclear reactor controlling systems and more particularly an apparatus for detecting trouble at each location in the nuclear reactor controlling system when a control rod for the nuclear reactor is operated.
Various means of the nuclear reactor output controlling systems are available and especially in the Boiling Water Reactor, the following two means are in general use, one employing the positional control of the control rod and the other controlling the flow rate of recirculating flow in the nuclear reactor core. In the latter, the variation in output by controlling the flow rate of recirculating flow is almost uniformly produced in the nuclear reactor core whereas in the former, variation in output due to the operation of the control rod is produced in the vicinity of the operated control rod in case of the boiling water nuclear reactor. The output variation locally produced in this manner tends to cause local overheating and it is necessary to take a greater care, as compared with the latter controlling the flow rate of recirculating flow, in the output controlling, control rod position supervising and local output supervising.
In a nuclear reactor of 500 MWe class, about one hundred control rods are used and since the output of nuclear reactor is determined dependent on inserted positions (depth) of the control rods, there are provided position detectors associated with respective control rods for detecting an inserted position of each control rod. The position detector does not effect continuous detection of the position but detects predetermined positions to produce an output. More particularly, a member movable along with the control rod is provided with a magnet and reed switches are located such that the magnet can be brought into linkage with these reed switches. The reed switches are fixed at positions along the moving direction of the control rod and equally spaced within the stroke of the magnet corresponding to from the extreme insertion position to the extreme withdrawal position of the control rod. The reed switches are fifty in number, for example, and only a reed switch fully linked with the magnet approaching thereto can produce an output. The magnet coupled with the control rod is not allowed to stop at all positions of the fifty reed switches, but is allowed to stop every other position. Herein, unity of driven distance of the control rod is termed as "notch." Accordingly, distance between two reed switches is equal to 1/2 notch and three reed switches are spaced by one notch. The control rod is allowed to stop every one notch.
50 REED SWITCHES ARE EMPLOYED FOR DETECTING THE POSITION OF ONE CONTROL ROD AS DESCRIBED ABOVE, AND THE TOTAL NUMBER OF REED SWITCHES USED IN THE NUCLEAR REACTOR AMOUNTS UP TO ABOUT 5000. Thus, a number of reed switches are prone to cause for failure in identifying the control rod position due to troubled reed switches during actual operation of the nuclear reactor. Hitherto, the control rod operation is manually effected and an operator can estimate the present position of the control rod, taking into consideration a local output and an amount of previous operation of the control rod, so that a continuous operation of the control rod was assured without affecting adversely on the safety operation of the nuclear reactor.
However, atomic power stations have increased in number today and they have shared a large portion of the total output of power stations. In this situation, there is arising need for intentionally varying loads of atomic power stations. To this end, the conventional manual operation of the control rods can not meet a quick response and automated operation is indispensable for controlling the control rods. In automated operation, simultaneous treatment of a number of information for judging the safety operation of the nuclear reactor is actually impossible and it is necessary to correctly know the present position of the control rod to be operated. Therefore, in the automated operation, when the present position of the control rod to be operated becomes unknown, identification of an amount of commanded operation with an amount of actual operation is prevented and unless controlled, the control rod moves randomly thus bringing the nuclear reactor into a very dangerous state. Further, there arise additional problems such as extreme difficulties in trouble judgement for automatic operation device and indistinctness of control rod reactivity worth.
In the automated operation, indistinctness of the control rod position gives rise to possible fatal accident and accordingly, in view of high security operation, the nuclear reactor should stop operating whenever only one of 5000 reed switches gets out of order. However, such stopage of the nuclear reactor in the event of failure of each reed switch and replacement of the troubled reed switch from the bottom of the reactor waste the availability of the nuclear reactor.
As has been described, it is of the most importance to know the inserted positions of all the control rods for automating the control rod operation.
On the other hand, the nuclear reactor controlling system has portions which must be supervised in connection with the movement of the control rod. Disengagement of the control rod poses a problem. Namely, the control rod is pushed upwardly toward the nuclear reactor core from the bottom thereof and a control rod body (neutron absorber) is connected to a hydraulic driving system through a connecting rod. Since the control rod body is sometimes replaced after subjected to a predetermined dosage of neutron flux and it is subjected to periodical maintenance and inspection, the control rod body cannot be constructed integrally with the connecting rod. Also the control rod body is so constructed as to be stopped every one notch in the nuclear reactor, as described above, and the magnet for detecting the position of control rod is carried on the connecting rod free from direct dose of radioactivity with the result that the control rod body alone is sometimes left in the nuclear reactor core and the position detector tends to indicate a different position. In such case, the control rod positioned at a different position from that indicated by the position detector causes reduction in the output of nuclear reactor, which reduction is compensated for by withdrawing another control rod. In contrast, the control rod hanging on the nuclear reactor core is caused to drop in the event that an earthquake strikes and as a result, a local output around the control rod is abruptly increased together with possible local overheating beyond the thermal threshold. In addition, the fuel assembly may be damaged by a shock due to the dropping of the control rod.
In the conventional nuclear reactor, control rods are withdrawn in an orderly manner to avoid a dangerous state in the event of the dropping accident and the sequence of withdrawal of control rods is supervised by means of a control rod reactivity worth minimizer. The minimizer is not protected completely from failure and an expedient for the detection of the disengagement has been desired.
Additionally, there are provided within the nuclear reactor core a plurality of local output detectors adjacent respective control rods. Outputs of the local output detectors are subjected to summation to produce a total output of the nuclear reactor while a local output of the detector is used as data for judging if the adjacent control rod is in condition to be withdrawn. In other words, a local output less than a predetermined value permits the withdrawal of control rod whereas a local output greater than the predetermined value blocks the withdrawal. Accordingly, in the event that the local output detector gets out of order, not only erroneous operation results but also transmission of erroneous data for supervising output distribution in the nuclear reactor and security of the reactor results.
As has been described, for automating the nuclear reactor controlling system, it is necessary to know correctly the inserted position of the control rods and to provide an apparatus capable of correctly judging abnormal state of the controlling system.